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Colonization and growth of Ascophyllum nodosum (L.) Le Jol. were investigated in two Maine estuaries from 1972 to 1978. Following denudation of intertidal rock, substrata were initially colonized by Fucus vesiculosus L.; eventually, Ascophyllum supplanted Fucus, and became dominant in terms of percentage cover. Ascophyllum settled first and most densely in the low intertidal zone, but its fastest growth occurred in the mid‐intertidal zone. Some, but not all, Ascophyllum germlings produced a vesicle within one year of colonization. The mean annual growth of A. nodosum was variable among sites, zones and years.

The regrowth of Ascophyllum nodosum and Fucus vesiculosus following harvest by various methods was studied from August 1973 to August 1976 et 6 sheltered and 2 exposed sites in Maine. Plants were cut at the holdfast (scraped) and at 15 and 25 cm above the substrate. At 5 of the 8 sites there were no significant differences among initial biomasses harvested by the three methods. Three successive annual harvests of A. nodosum at the 15 and 25 cm levels yielded successively lower biomasses. The biomass of F. vesiculosus increased in scraped areas with successive reharvests but remained relatively constant in areas cut at 15 and 25 cm. When present, Littorina littorea retarded or prevented recovery of both species in scraped areas. Recovery was faster in sheltered areas than in exposed regions, but was dependent to a large extent on the morphology and age structure of the Ascophyllum populations.

Ascophyllum nodosum (L.) Le Jol., and Chondrus crispus Stackh., from protected rocky shores permitted colonization and development of F. vesjculosus throughout the intertidal region. Following colonization, the mortality of F. vesiculosus gerrnlings was high. Such losses were not reflected in area1 cover measurements, however, because of the continued growth of surviving thalli. Mortality of large plants occurred mainly during winter, owing to ice and storm damage. This mortality, as well as a reduced growth rate, was responsible for the slow increase in algal cover during winter.